That's a very good attempt at Van Gogh's Sunflowers. When I was 12 years old I did my own attempt with a craft knife and oils, so I know the picture. You can actually see where your young lad thought it was all a little too complicated for him. Or perhaps the teacher was saying that time was tight. Either way, you can see how well your 6 year old managed to get close to the spirit of the picture.
Exactly my point however 6 year old concentration span is probably more to blame. I would think he had the main bit done but then started to wander and splashed on the rest

It's funny because he is now copying things from my Picasso books because he says they are easy to copy. He misses the meaning and intention behind what Picasso did and just sees the triangles, squares and shapes fitting together however he is interested, he is proud of what he does and I encourage him.
My youngest is nearing 6 years old so only a few months younger now than when my eldest did the sunflowers copy, however my youngest is still doing barely recognisable scrawls and matchstick (or rather sausage shaped) men. Mr Gecko would say that's crap son, give up Art you'll never make it while my approach is to encourage him, and praise hiim and the sausages are getting better

proper Lincolnshire sausages now.
Who's to say which of the 2 will be the better artist when they are older?
If it were an adult we
could say that's crap mate why are you wasting your time, not for the child though, they are developing at a much faster rate and in spurts.
Also if it were an adult bear in mind you may choose to be diplomatic as the adult may well punch you on the nose and tell you to **** I mean clear off you irritating Irk.